Zeolitic molecular sieves have been utilized in a variety of catalytic and adsorption processes. When the molecular sieves are used to process hydrocarbon feeds, often after a period of use, a carbonaceous deposit material forms on the molecular sieve which imparts some loss of activity. Usually, the activity can be at least partially restored by performing an oxidative regeneration which removes a portion of the carbonaceous deposit material from the molecular sieve.
When the hydrocarbon feeds contain sulfur compounds, the carbonaceous deposit material that forms on the molecular sieve can also contain sulfur. Upon oxidative regeneration, the sulfur is converted to sulfur oxides, i.e., primarily SO.sub.2 and some SO.sub.3, and expelled from the zeolitic molecular sieve with the regeneration gas. Since it is often advantageous to recycle the regeneration gas through the molecular sieve in a closed loop, these sulfur oxides are repeatedly contacted with the molecular sieve.
Despite the restorative effect of the oxidative regeneration with respect to the carbonaceous deposit material, it has been observed that repeated contact with sulfur oxides can cause permanent deactivation of the molecular sieve. It appears that during regeneration, the SO.sub.2 is further oxidized to SO.sub.3 at the relatively high regeneration temperatures, i.e., typically, 750.degree.-1000.degree. F., when contacted with the zeolitic molecular sieves. The SO.sub.3 then apparently irreversibly reacts with the molecular sieve and causes the above-described deactivation. The exact mechanism of the deactivation is not known although it is generally observed as either a loss in adsorptive capacity or a loss in catalytic activity.
This problem can be encountered in essentially any catalytic or adsorption processes that utilize zeolitic molecular sieves to process sulfur-containing hydrocarbon feeds and which may be adversely affected by exposure to sulfur oxides during oxidative regenerations.
For instance, U.S. Pat. Nos. 3,700,589 and 4,176,053 describe processes for separating normal paraffins from non-normal paraffins in vapor phase using a fixed adsorption bed containing 5A zeolitic molecular sieve adsorbent. The hydrocarbon streams treated in accordance with the above-identified patents consist essentially of mixtures of branched chain paraffins and normal paraffins boiling in the gasoline and kerosene ranges. Such mixtures occur as petroleum naphthas, both light and heavy, natural gasolines and natural gas condensates, but also can be the products of processes outside the petroleum production and refining industry. In general, the hydrocarbons of these streams contain from about 4 to about 13 carbon atoms and can contain sulfur compound impurities typically in a concentration of less than 400 ppmv. The processes of the above-identified patents provide for an oxidative regeneration to remove carbonaceous deposit material that gradually accumulates on the adsorbent and causes a reduction in adsorption capacity, but do not specifically provide for the removal of sulfur oxides formed during the regeneration process.
Another patent, U.S. Pat. No. 3,422,005 describes a separation process for separating normal paraffins from a hydrocarbon vapor feed stream having 10 to 25 carbon atoms per molecule using normal hexane purge and zeolitic molecular sieve adsorbent. This carbon range covers both kerosene and gas oil feeds which have ASTM boiling ranges of from about 275.degree.-600.degree. F. and about 400.degree.-700.degree. F., respectively and commonly contain sulfur compounds in concentrations as high as 3000 ppmv.
In view of the fact that sulfur oxides can cause permanent deactivation of zeolitic molecular sieves during oxidative regenerations, despite the fact that the organic sulfur compounds typically present in the hydrocarbon feeds apparently do not cause permanent deactivation, it can be appreciated that it would be advantageous to remove sulfur oxides from regeneration gas before recycling it to the molecular sieve.
There are a variety of well known processes for removing sulfur oxides from gaseous streams. For example, in flue gas desulfurization, wet scrubbing processes have been employed wherein the flue gas is contacted with an aqueous solution of an organic acid to form a soluble sulfite or sulfate which is thereafter removed from solution by reaction with a calcium compound such as calcium hydroxide. Other flue gas desulfurization processes, such as described in U.S. Pat. Nos. 4,600,568 and 4,604,269, involve spraying a dry absorbent, such as slaked lime, into a flue gas stream along with an aqueous solution containing a solubilizing agent and thereafter removing the loaded absorbent by using a bag filter or electrostatic precipitator. While the above-identified methods are effective, they are far too complex for the purposes of the present invention.
U.S. Pat. No. 4,551,304 describes a method of purifying air loaded with pollutants, such as sulfur dioxide, nitrogen dioxide, nitrogen oxide and hydrocarbon compounds, wherein soda-lime absorbent is used to remove the acid gases from the air. The patent discloses the preferred initial step of ozonizing the air to convert SO.sub.2 to SO.sub.3 or NO to NO.sub.2, such converted compounds being more readily absorbed on the soda-lime.
U.S. Pat. No. 3,812,200 discloses a process for the reactivation of a soda-lime absorber bed used for removal of H.sub.2 S and CO.sub.2 from normally gaseous hydrocarbons, i.e., propane and propylene, by injecting steam into the bed after it has reached its capacity. The above-identified patent discloses that after steam reactivation, the absorber can be utilized again to have more than 50% additional on-stream time.
None of the above-identified sulfur removal processes specifically address the problem of inhibiting deactivation of zeolitic molecular sieves by removing sulfur oxides from the regeneration gas streams. Accordingly, methods are sought for the regeneration of zeolitic molecular sieves having a sulfur-containing carbonaceous material deposited thereon wherein sulfur oxides are removed prior to contacting with the molecular sieves in order to inhibit deactivation thereof.